This invention relates generally to handling devices for web or sheet material. More particularly, the invention relates to a stretch frame for tensioning or stretching such materials edgewide.
As will appear from the following description, the stretch frame of the invention may be employed for various uses. Its primary application is in the silk screen printing industry for supporting the printing screen in a taut condition. In this industry, the frame is commonly referred to as a screen chase. The invention will be described in relation to its use as a silk screen chase.
In the silk screen printing process, also known as "silk-screening", the image to be printed is permanently formed on a printing screen by a photo etching process or the like which blocks the screen openings in a way which defines the image. Ink is then forced through the unblocked screen openings onto a printing surface to reproduce the image on the surface. Precise positioning of the screen is essential to achieve high quality results with good registration and resolution. Thus the screen must be maintained taut and in a stable position when the ink is forced through it. In some cases a stainless steel mesh screen has been utilized to minimize stretching. Nylon screens are now more commonly used because they are less expensive. The use of relatively elastic nylon makes it even more important than previously to provide a structure which will maintain the screen in a stable taut condition.
A variety of stretch frames or screen chases for this purpose are known. This invention is connected with frames of this type which use tensioning rollers which grip the screen edges and are rotated to stress the screen. Some early frames of this kind fail to securely grip the silk screen because they utilize screen tensioning rollers which grip the screen edges only at spaced locations. A solution to this problem was provided by the "keystone" structure, also referred to as an anchor bar structure, described in the applicant's earlier U.S. Pat. No. 3,962,805. Other frame improvements are described in the applicant's U.S. Pat. Nos. 3,601,911; 3,482,343; 3,553,862 and 3,608,854 and a pending application Ser. No. 132,389 U.S. Pat. No. 4,338,860 which was filed on Mar. 21, 1980. Roller anti-rotation means in the form of ratchet constructions for holding stretch frame rollers against reverse rotation to release the sheet tension are described in U.S. Pat. No. 3,908,293. These particular ratchet constructions act only on the roller ends and, for this reason, provide inadequate support for the roller.
A primary object of the invention is to provide an improved stretch frame for silk screening and other applications.
Another object of the invention is to provide an improved screen chase which will positively hold the printing screen so as to prevent its movement during the printing process.
Yet another object of the invention is to provide an improved stretch frame or screen chase which is more durable than the frames generally available prior to this time.
Still another object of the invention is to provide an improved stretch frame or screen chase which is more simple in construction than the frames generally available.